Sun

• 1. Introduction

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  The Sun fuses about 600 billion kilograms of hydrogen into helium every second. In that same second, 4 billion kilograms of matter vanish, converted entirely into energy. This furnace at the centre of the Solar System makes up about 99.86% of the total mass of everything orbiting it. It formed approximately 4.6 billion years ago, and it is now roughly halfway through its main-sequence life. From Earth it sits about 8 light-minutes away, a distance so foundational that astronomers turned it into a unit of length. How does a sphere of hot plasma hold itself together against its own staggering output? Why is its outer atmosphere hundreds of times hotter than its visible surface? And what happens to the planets when this star finally runs out of fuel? The answers reach from the rotating core to neighbouring stars that may be the Sun's lost siblings.

• 2. Furnace At The Core

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  Close to 15.7 million kelvin: that is the temperature at the centre of the Sun, where the proton-proton chain turns hydrogen into helium. The core extends from the centre to about 20 to 25% of the solar radius, with a density up to 150 times that of liquid water. It is the only region producing an appreciable amount of thermal energy through fusion. About 99% of the Sun's power is generated within the innermost 24% of its radius, and almost no fusion occurs beyond 30%.

  Fusing four protons into a single helium nucleus releases around 0.7% of the fused mass as energy. That conversion drives the mass-energy rate of 4.26 billion kilograms per second, equal to 384.6 yottawatts, or about 9.192 megatons of TNT every second. Yet the energy production per cubic metre is modest. Theoretical models indicate a maximum power density of approximately 276.5 watts per cubic metre at the centre of the core. According to Karl Kruszelnicki, that is about the same power density found inside a compost pile.

  The fusion rate sits in a stable equilibrium. A slightly higher rate would heat the core and expand it against the weight of the outer layers, reducing the density and bringing the rate back down. A slightly lower rate would cool and shrink the core, increasing the density and pushing the rate back up. Currently 0.8% of the Sun's energy comes from a separate set of reactions called the CNO cycle, a proportion expected to grow as the Sun ages and brightens.

• 3. The Long Climb Outward

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  A million years. That is roughly how long a photon takes to cross the radiative zone, the thickest layer of the Sun, scattering off dense gas again and again. The zone begins above the core at about 0.25 solar radii and reaches out to about 0.7 solar radii. Across that span the temperature drops from approximately 7 million to 2 million kelvins, and the density falls a hundredfold, from 20,000 to 200 kilograms per cubic metre.

  Above the radiative zone lies a thin transition layer called the tachocline, where the uniform rotation below meets the differential rotation above. This shear, with successive horizontal layers sliding past one another, is where a solar dynamo is hypothesised to generate the Sun's magnetic field. Beyond it, the convection zone runs from 0.7 solar radii out to near the surface. Here plasma heated at the tachocline expands, rises, cools beneath the surface, and sinks again in an orderly cycle of thermal cells.

  High-energy gamma ray photons born in fusion are absorbed by the radiative zone's plasma after travelling only a few millimetres, then re-emitted in random directions at slightly lower energy. Estimates of the total photon travel time to the surface range between 10,000 and 170,000 years. Neutrinos make the same journey in just 2.3 seconds, because they almost never interact with matter. That ghostly behaviour once produced a famous puzzle. Measurements found only a third of the expected electron neutrinos, and the 2001 discovery of neutrino oscillation resolved it: the neutrinos had simply changed flavour before reaching the detectors.

• 4. Surface And Atmosphere

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  5,772 kelvin: the photosphere radiates roughly like a black body at that temperature, interspersed with atomic absorption lines from the tenuous layers above. The photosphere is the visible surface, the layer below which the Sun becomes opaque to visible light. It is only tens to hundreds of kilometres thick and slightly less opaque than air on Earth. Because its upper part is cooler than its lower part, the disk looks brighter in the centre than at the limb, a phenomenon called limb darkening. The convection zone's thermal columns imprint a granular pattern on it, the solar granulation, formed by roughly hexagonal Benard cells.

  The coolest part of the Sun sits just above, a temperature minimum region at about 4,100 kelvin, cool enough for simple molecules like carbon monoxide and water to exist. Above that lies the chromosphere, around 2,000 kilometres thick, named from the Greek root chroma for colour because it appears as a coloured flash during total solar eclipses. Its temperature climbs to around 20,000 kelvin near the top, where helium becomes partially ionised.

  The corona presents one of solar physics' deepest riddles. While the photosphere sits near 6,000 kelvin, the corona reaches 1,000,000 kelvin, and its hottest regions run from 8,000,000 to 20,000,000 kelvin. The high temperature means it cannot be heated by direct conduction from the photosphere. Two mechanisms have been proposed: wave heating from turbulence in the convection zone, and magnetic heating released through magnetic reconnection in flares and smaller nanoflares. Since all waves except Alfven waves dissipate before reaching the corona, and Alfven waves do not easily dissipate there, research has shifted toward flare heating. In April 2021, the Parker Solar Probe crossed the corona's outer boundary, the Alfven critical surface, at heliocentric distances of 16 to 20 solar radii.

• 5. The Magnetic Heartbeat

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  A quasi-periodic 11-year cycle governs how the number and size of sunspots waxes and wanes. Sunspots are dark patches on the photosphere where magnetic field concentrations inhibit convective heat transport, leaving them slightly cooler and therefore darker. The largest can stretch tens of thousands of kilometres across. The Sun's polar field measures 1 to 2 gauss, but reaches around 3,000 gauss inside sunspots. The 11-year sunspot cycle is half of a 22-year Babcock-Leighton dynamo cycle, an exchange of energy between toroidal and poloidal magnetic fields, with sunspot polarity alternating each cycle under Hale's law.

  This activity drives solar flares and coronal mass ejections at sunspot groups, while high-speed solar wind streams pour from coronal holes. On Earth, the effects include auroras at moderate to high latitudes and disruption of radio communications and electric power. The cycle is not always reliable. In the 17th century, sunspots nearly vanished for several decades during the Maunder minimum, a span that coincided with the Little Ice Age, when Europe experienced unusually cold temperatures.

  The magnetic field reaches far past the Sun itself. The electrically conducting solar wind carries it outward into the interplanetary magnetic field, and the Sun's rotation twists it into an Archimedean spiral called the Parker spiral. The solar wind keeps flowing through the heliosphere until it meets the heliopause more than 50 astronomical units away. The Voyager 1 probe passed through that boundary on the 25th of August 2012, at approximately 122 astronomical units from the Sun, registering a surge in cosmic ray collisions and a drop in solar wind particles.

• 6. A Sphere Almost Perfect

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  8 parts per million: that is the measured oblateness of the Sun, the relative difference between its equatorial and polar radii. Precise measurement required satellites, since atmospheric distortion ruled out ground observation. When the Solar Dynamics Observatory and the Picard satellite delivered high-precision values, the result of 8.2 times ten to the minus six was even smaller than expected. Those measurements determined the Sun to be the natural object closest to a perfect sphere ever observed. The oblateness stays constant regardless of changes in solar irradiation, and the tidal pull of the planets does not significantly affect the Sun's shape.

  The roundness once carried a heavier theoretical burden. Its oblateness was proposed as sufficient to explain the perihelion precession of Mercury. Einstein countered that general relativity could account for the precession using a perfectly spherical Sun, and the later measurements supported a Sun rounder than the old hypothesis required.

  The Sun also spins unevenly. It rotates faster at its equator than at its poles, a differential rotation driven by convective motion and the Coriolis force. In a frame defined by the stars, the period is about 25.6 days at the equator and 33.5 days at the poles. A survey of solar analogues suggests the early Sun rotated up to ten times faster than today, with greater X-ray and UV emission, before magnetic braking slowed it. A vestige of that primordial speed survives in the core, which still rotates about once per week, four times the mean surface rate.

• 7. Birth, Death, And Siblings

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  4.567 billion years: the radiometric date of the oldest Solar System material, consistent with the Sun's estimated age of about 4.6 billion years. The Sun formed from the collapse of part of a giant molecular cloud of mostly hydrogen and helium. Ancient meteorites carry traces of short-lived isotopes such as iron-60, which form only in exploding stars, pointing to one or more nearby supernovae. A shock wave from such a supernova would have compressed the cloud and triggered the collapse. As one fragment fell inward it spun up by conservation of angular momentum, and surplus gas and dust formed a protoplanetary disk that became the planets. Two stars, HD 162826 and HD 186302, share similarities with the Sun and are hypothesised to be its stellar siblings from the same cloud.

  The Sun's death is mapped in striking detail. In about 5 billion years core hydrogen fusion will stop, and the contracting core will swell the Sun first into a subgiant, then a red giant exceeding 1,000 times its present luminosity. It will engulf Mercury and Venus, and at the tip of the red-giant branch, 7.59 billion years from now, it will swallow Earth too. By then the Sun will be about 256 times its current size, with a radius of 1.19 astronomical units. A violent helium flash follows, converting 6% of the core into carbon within minutes through the triple-alpha process.

  The final acts come faster. After the asymptotic-giant-branch phase, with thermal pulses pushing luminosity as high as 5,000 times today's level, the Sun ejects half its mass as a planetary nebula. The exposed core reaches over 100,000 kelvin as a white dwarf containing an estimated 54.05% of the Sun's present mass. That nebula disperses in about 10,000 years, while the white dwarf survives for trillions of years before fading toward a hypothetical black dwarf giving off negligible energy.

• 8. Measuring The Wanderer

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  1684: the year Giovanni Domenico Cassini determined the first reasonably accurate distance to the Sun. Knowing direct solar parallax measurements were difficult, he measured the parallax of Mars instead. He sent Jean Richer to Cayenne in French Guiana for simultaneous readings while he observed from Paris, then applied Kepler's laws to find the Earth-Sun distance. His value ran about 10% smaller than modern figures, yet far larger than every earlier estimate. Observations of the 1769 transit of Venus later pinned the average distance to within 0.8% of the modern value.

  The path to that precision was long. The Greek philosopher Anaxagoras reasoned the Sun was a giant flaming ball of metal larger than the Peloponnesus, and that the Moon reflected its light. Aristarchus of Samos first proposed in the 3rd century BC that the Sun sits at the centre with the planets orbiting it, a view Nicolaus Copernicus developed into a detailed model in the 16th century. The source of the Sun's energy stayed a puzzle far longer. Lord Kelvin and Hermann von Helmholtz proposed gravitational contraction, but it gave an age of only 20 million years, far short of geological evidence. Albert Einstein's mass-energy relation supplied the clue, and in 1920 Arthur Eddington proposed that core fusion of hydrogen into helium powered the star.

  The study of helium itself began at the Sun. In 1868, Norman Lockyer hypothesised that unexplained absorption lines came from a new element he named helium, after the Greek Sun god Helios. Twenty-five years later, helium was isolated on Earth. The Sun has long been more than a science problem. It was worshipped as Ra in Egypt, as Surya in Hinduism, as Utu among the Sumerians, and celebrated as Sol Invictus in the late Roman Empire soon after the winter solstice, a festival that influenced Christmas.